add animated version of mixer gm

Bug: skia:
Change-Id: I8ebb953817baae603c311ef0c980d6d94bbb655a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201689
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>